INTRODUCTION". 21 pounds per square inch higher than if determined by the parallel-sided test. (3) Flat bars differ from rounds in having less tensile strength, lower elastic limit, lower elastic ratio, greater elongation, and a slightly lower reduction of area. (4) In testing flats the elongation increases regularly as the width increases, while the reduction of area regularly decreases. (5) The percentage of elongation decreases as the length of -the test piece increases. The law of change is such that if a piece 8 inches long gives 30 per cent, elongation, a piece of infinite length would give about 24 per cent. THE INFLUENCE OF CEKTAIN ELEMENTS UPON STEEL. _ Nothing is more difficult than to state accurately the effect of different elements upon the strength and ductility of steel. Those who have studied and worked over the problem differ among themselves and differ widely. Yet it is a common thing for engineers to write a specification calling for a steel of a certain tensile strength, and limiting the content of carbon, phosphorus, manganese and sulphur. It often happens that such specifications are impracticable, if not impossible. For instance, the tensile strength is allowed to vary between 60,000 pounds and 70,000 pounds per square inch, but it may be that the highest allowable contents of carbon, phosphorus and manganese will actually give a strength of only 65,000 pounds. Now it will be evident that the true allow-ance of tensile strength is not 10,000 pounds, but 5000. pounds. It".is also evident that the manufacturer must keep his phosphorus and manganese at the highest point, a thing the engineer is very far from wishing, but which he has ignorantly made -necessary. The slightest consideration will show that it is a mathematical impossibility for the engineer to put both chemical and physical limits and have them coincide, unless he knows absolutely the effect of each element upon the strength of steel, and no man in the world claims to know that to-day. It is right for the engineer to specify certain parts of the chemical formula, but he must leave room for the manufacturer to attain the physical results. If he specifies the phosphorus limit, he should leave the carbon open, and if he specifies the carbon he should leave the phosphorus and manganese to the manufacturer.